Fix failure-remedy hint, orphan run dirs, and close documentation gaps

[ali-abassi]

Addresses the five clarity findings from the external-review pass.

Code

• Failure hint contradicted the docs (run_steps.py:1451): a failed durable run printed a site-packages run_steps.py path with undocumented --from/--run-dir flags. It now prints the documented piw resume WORKFLOW RUN_DIR remedy (legacy non-durable runs keep the low-level hint). Covered by an assertion in the recovery test.
• Orphan run dirs: running a required-input workflow without --input created a runs/<id>/ directory stuck in initialized. The guard now fires before any directory is created. New regression test.

Docs

• Parallelism documented: workers: (default 4, max 16), needs: as the only serialization guarantee — README bullet, USAGE §5, ARCHITECTURE.
• Previously undocumented schema features: judge, qa, cwd, preview, retry_jitter, retry_max_delay_seconds (USAGE, verified against schema + runner).
• README: real run-bundle listing + actual ledger.json cost line; run directory location; piw expanded; LangGraph/Temporal/Prefect/Make positioning table; 'hosted workers' → 'hosted worker fleets'.
• USAGE: input.description requirement stated; examples README notes gitignored runs//approvals.

Verification

• ruff check src scripts tests clean; unittest: 29 tests OK (was 28).
• Live: recovery example fails → printed remedy pasted verbatim → run completes; missing-input run exits 1 with no runs/ dir; quickstart (create/run hello) unchanged.